In recent years, smaller and lighter mobile data terminals such as mobile phones, notebook personal computers, and smartphones have been increasingly used and batteries used as driving power supplies therefor have been required to have higher capacity.
In particular, nonaqueous electrolyte secondary batteries, which are charged and discharged in such a manner that lithium ions move between positive and negative electrodes, have high energy density and high capacity and therefore are widely used as driving power supplies for the above mobile data terminals.
Furthermore, the nonaqueous electrolyte secondary batteries are recently attracting attention as power supplies for power for electric vehicles and the like and applications thereof are expected to be further expanded.
In such applications, the following improvements are required: further improvements in high capacity enabling batteries to be used for a long time, in the reduction of battery cost, and in the safety in the case where force is applied from the outside by dropping or the like.
One of methods for evaluating the safety of nonaqueous electrolyte secondary batteries is an impact test in which a certain impact is applied to a battery from the outside by vertically dropping a predetermined weight on the battery in charge and no occurrence of rupture or ignition is confirmed.
Patent Literature 1 describes a technique for preventing the ignition and rupture of a battery in such a manner that an uncoated portion is provided on the outermost periphery of a negative electrode, a negative electrode current-collecting tab provided on the outermost periphery and the uncoated portion are arranged so as to overlap each other, and the short-circuit current and heat caused by an external impact are thereby effectively diffused.